Sealing system

ABSTRACT

A piston reciprocally movable within a cylinder is sealed to the cylinder wall by fluid at a pressure greater than those of the fluids on the opposed sides of the piston supplied to the space (18) between two seals (19, 20) operative between the piston and cylinder wall, an accumulator or auxiliary intensifier chamber (67) being provided from which overpressure fluid is supplied to the space between the two seals to regulate the pressure between them. The two seals between the piston and the cylinder wall can be constituted respectively by first (19A-19C) and second (20A-20C) groups or pluralities of separate seal rings selected as regards material and dimensions so as to perform specific functions. Also, a sealing system between a piston and a cylinder within which the piston is reciprocally movable can comprise a forward seal (19, 20), conveniently having two seals or seal ring groups, separating a process or working fluid (4) being moved by the piston, from a second fluid (7) in the space around the piston, and a rearwardly spaced barrier seal (21A-21C) separating the second fluid from a third fluid filling a housing (6) containing a motor by which the piston is driven.

The invention relates to sealing systems for members which arereciprocably movable within a cylinder.

In many types of equipment, a piston or like member is reciprocablymovable in a cylinder, its two opposed sides being exposed to differentfluid pressures. This pressure differential produces a tendency forleakage between the reciprocable member and the cylinder wall of fluidfrom the higher pressure side to the lower. Such leakage can bedisadvantageous where the lower pressure fluid would be contaminated byadmixture with the other, or where the other fluid contain abrasivematerial which needs to be excluded from the lower pressure fluid inorder to protect other parts of the equipment with which that fluid isin communication.

The invention is accordingly concerned with the provision of an improvedsealing system for a member reciprocably movable within a cylinder.

There is known from EP-A-0 314 493 an effective sealing system for amember reciprocably movable within a cylinder of the kind in whichoverpressure fluid, that is, fluid at a pressure greater than those ofthe fluids on the opposed sides of the member, is supplied to the spacebetween two seals operative between the member and the cylinder wall.The overpressure of the fluid is obtained by constructing thereciprocable member in two portions capable of relative axial movement,the portions defining an intensifier chamber between them into whichfluid is drawn through non-return valve means on relative movement apartof the two portions and from which the fluid is urged into the spacebetween the seals by movement of the portions together.

In accordance with a first aspect of the invention, there is provided,in a sealing arrangement of this kind, an accumulator or auxiliaryintensifier chamber from which overpressure fluid is supplied to thespace between the two seals when the pressure in the main intensifierchamber drops.

It will be evident that the pressure within the intensifier chambervaries during the cycle of movement of the two portions of the member,falling at the end of each stroke, thereby reducing the effectiveness ofthe sealing arrangement at critical times during the cycle. Theprovision of the auxiliary intensifier chamber in accordance with theinvention provides a reserve overpressure which tends to regulate theeffective fluid pressure in the space between the two seals.

A sealing system embodying the present invention thus convenientlyincludes passage means leading from the intensifier chamber to theaccumulator and the space between the seals by way of a non-return valvepermitting flow from the intensifier chamber only. Biassing meanstending to reduce the volume of the accumulator is provided.

When the two portions of the reciprocable member are moving togetherduring a compressive stroke, the accumulator is expanded by pressurefluid from the intensifier chamber. The pressure in this chamber thenfalls as the two portions of the member commence the return stroke, andthe accumulator pressure is applied to the space between the seals,under the biassing means, the non-return valve isolating the space andthe accumulator from the intensifier chamber.

In accordance with a second aspect of the invention, there is provided asealing arrangement of the kind described in which the two seals betweenthe member and the cylinder wall are constituted respectively by firstand second groups or pluralities of separate seal rings.

The seal rings of the first and second groups of sealing rings, whichare conveniently carried by the reciprocable member, are selected asregards material and dimensions so as to perform specific functions.Preferably, the seal ring located at the forward end of the reciprocablemember is such as to prevent passage of particles in the process orworking fluid to the next following seal ring or rings, so that theoperating conditions for such ring or rings are improved. The forwardseal ring can thus be a segmented ceramic ring or two such ringsreceived in a common groove. The following ring or rings have, inaddition to sealing, a scarping function and also an oil distributionfunction where the fluid to be isolated from the process fluid is alubricating oil. The following ring or rings then allow sufficientlubricant to pass to the forward seal ring, whilst minimising theconcentration of process fluid flowing in the opposite direction to thelubricant flow.

The second group of seal rings, rearward of the pressure intensifieroutlet port, comprise two or more rings selected to have characteristicssuch as to minimise flow from the intensifier chamber away from theprocess fluid.

In accordance with a third aspect of the invention, there is provided,between a member and a cylinder within which the member is reciprocablymovable, a sealing system comprising a forward seal separating a processor working fluid being moved by the member from a second fluid in thespace around the member, and a rearwardly spaced barrier seal separatingthe second fluid from a third fluid.

Typically, the third fluid fills a housing containing a motor, forexample, a linear electric motor, by which the reciprocable member isdriven, and the second and third fluids can then be the same. Theforward seal is preferably, but not necessarily, of the multiple sealring kind described herein, advantageously, but not necessarily, inassociation with a pressure intensifier device of the kind described inEP-A-0 314 493, which can but need not include an accumulator also asdescribed herein.

The barrier seal can comprise three seal rings spaced to define twochambers between them, of which the forward chamber communicates with arelatively high pressure fluid source, and of which the rear chamber isventilated or evacuated or pressurized at a pressure lower than that inthe forward chamber and that of the third fluid.

Although a piston pump, particularly if provided with a sealingmechanism in accordance with the first and second aspects of theinvention, can operate satisfactorily in appropriate circumstances,there is a risk of pump or motor damage as a result of a single minorfailure of the cylinder, the piston or the seal rings. This arisesbecause large quantities of the process fluid could enter the motorhousing undetected.

The barrier seal eliminates such problems. Coating failure or failure ofthe pressure intensifier mechanism can be detected immediately bymonitoring flush fluid flow and pressure in the fluid feed line betweenthe intensifier mechanism and the barrier seal. This trapped fluid herein case of any coating or piston failure reason will experience apressure increase that can be sensed and used to effect a shut-down. Inaddition the oil flow to this area can be closely monitored. Thus theeffect of the barrier seal is to give extremely good leakage ratecontrol and its use consequently indicates a long performance before apossible pressure rise should take place.

The invention is further described below, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic sectional side view of a piston pump embodying thepresent invention;

FIG. 2 is a schematic sectional side view of a piston pump similar tothat of FIG. 1 but modified for subsea use;

FIG. 3 is a fragmentary schematic sectional side view of sealingarrangements which can be incorporated in the piston pump of FIG. 1 orFIG. 2;

FIG. 4 is a partial sectional side view of a modified form of a pistonpump in accordance with the present invention; and

FIG. 5 graphically displays how pressures within the piston pump of FIG.4 vary during a pumping stroke.

In the drawings, like or functionally equivalent parts in differentfigures are designated by the same reference numerals. To facilitateunderstanding, spacing is shown between the various sealing meansillustrated and one of the surfaces to which a seal is made in practice.

The piston pump shown in FIG. 1 comprises a cylinder 1 in which a pistonhead 2 is slidably received. To the left (as shown) of the piston head2, the cylinder 1 provides a variable volume process or working chamber4 associated with appropriate inlet and outlet valves (not shown) sothat a process or working fluid is moved into and out of the chamber byreciprocal movement of the piston head 2.

The piston head 2 is reciprocably driven within the cylinder 1 by apiston rod 5 which extends to the right (as shown) into a motor housing6, in which is received a prime mover (not shown) for effectingreciprocation of the piston rod. The prime mover can convenientlycomprise a linear electric motor having a driving or stator winding onthe motor housing wall co-operating with a driven winding carried by thepiston rod. The piston rod 5 is spaced from the cylinder wall to providea chamber 7 into which lubricant oil is fed from a supply sourcesuitably pressurised by way of a feed line 8 through a port 9 in thecylinder wall at a position spaced away from the piston head.

The forward or driving end of the piston rod 5 has a reduced diameterend portion 10 received within a cylindrical recess formed at theadjacent end of the piston head 2, with an O-ring 11 or other suitabletype of seal between the end portion and the wall of the recess. Avariable volume intensifier or overpressure chamber 12 is thus formedbetween the piston head and the piston rod. A passage 14, comprisingtransverse portions in the piston rod end portion 10 adjacent the mainbody of the piston rod, and an axially extending portion, communicateslubricant oil in the feed chamber 7 with the intensifier chamber 12. Acheck or non-return valve 15 in the axial passage permits fluid flowinto the chamber 12 but prevents return flow. Transverse passages 16 inthe wall of the piston head 2 connect the chamber 12 to ports 17 at theexterior of the head. The ports 17 are located between an outer orforward group 19 of seal rings and an inner or rear group 20 of sealrings defining a space 18 between them, the piston head 2 and thecylinder 1.

During the working or discharge stroke, leftward movement of the pistonrod 5 is transmitted to the piston head 2, by way of lubricant oil inthe chamber 12. Because of the valve 15, the fluid is pressurisedaccordingly, so that intensified pressure or overpressure iscommunicated to the space 18 between the inner and outer groups 19,20 ofseal rings through the transverse passages 16. Return movement of thepiston rod 5 on the suction stroke reduces pressure within the chamber12 and lubricant oil enters the chamber 12 through the passage 14 andthe non-return Valve 15. Limited relative reciprocation of the pistonhead 2 and piston rod 5 consequently enhances the seal provided by thesealing groups 19,20.

The piston pump of FIG. 1 also incorporates a barrier seal system shownat the right-hand side of the figure which co-operates with the sealarrangements associated with the piston head 2.

The barrier seal system comprises three seal rings 21A, 21B & 21Clocated in grooves in the cylinder wall between the port 9 and the motorhousing 6 and spaced to define forward and rear chambers 22 and 24between them.

The chamber 22 between the seal rings 21A,21B communicates with apressurised barrier fluid line 25 through a port 26 in the cylinderwall. The chamber 22 thus receives a barrier fluid, which canconveniently be the lubricant oil supplied to the feed chamber 7 throughthe port 9, but at a pressure greater than that in the lubricant oilfeed line 8. The chamber 22 is thus pressurised to a greater pressurethan either the feed chamber 7 and the rear barrier chamber 24 but anegative flow into the line 25 can occur due to the positive scopingfunction that takes place in chamber 22.

The chamber 24 is ventilated, or filled with gas at low pressure, thiscondition being maintained by vacuum or low pressure generated by amotor driven pump unit 27 supplied at a suction port 29 in the cylinderwall. Alternatively, the chamber 24 can be filled with the barrier fluidor oil at a lower pressure than prevails in the chamber 22 and in themotor housing 6.

The three barrier seal rings 21A, 21B & 21C thus act as barriers toeliminate any penetration of the process fluid into the motor housing 6.The separation of the barrier seal system from the motor housing 6,which is oil filled through a port 30 in the cylinder wall connecting toa feed line, prevents penetration into the motor housing 6 of anymicrofilm at the cylinder surface wall which may be contaminated byprocess fluid.

Use of a barrier seal system as illustrated affords a high degree ofoperational safety. The various seal rings employed can give 100%redundancy, and the risk of pollution by the process fluid iseliminated. The cylinder, piston and seals can be inspected, serviced orreplaced without draining oil from the motor housing. In case of apiston failure, the pump can be run as plunger to avoid immediateshut-down. Production can continue until a planned service shut-down canbe performed.

The piston pump of FIG. 2 corresponds to that of FIG. 1 except formodifications appropriate to use as a sub-sea pump, for example in afluid hydrocarbon extraction system. Only the modifications will bedescribed.

The pump is received within a housing 35 forming part of a submergedinstallation connected to surface equipment indicated schematically at36 by suitable supply and delivery lines including the illustrated lines37 & 39. Single seal rings 19 & 20 replace the seal ring groups of FIG.1.

By way of the line 39, barrier oil is conveyed from a supply source inthe form of a reservoir 40 at the surface equipment 36 to the interiorof the housing 35 at 41 and from the housing interior to the rearbarrier chamber 24 through a passage 44 containing a restriction 45which provides the required pressure differential across the seal ring21c. The pump housing interior can communicate with the interior of themotor housing 6, from which the passage 44 can extend. By way of theline 37 the barrier oil is returned to a return compartment 46 at thesurface equipment 36 from which the oil can re-enter the reservoir 40 byway of a flush compartment 47.

The line 39 also conveys the barrier oil to the forward chamber 22through a branch line 50 including a non-return valve 51 to preventback-flow from the chamber. The chamber 7 is fed from the chamber 22through a passage 54 containing a restriction 55 providing a pressuredifferential between the chambers and across the seal ring 21B. Thearrangements illustrated and described make it possible to assess thequantity of barrier oil flowing into the process fluid, by monitoringthe flow and return of the barrier oil through lines 39 & 37respectively.

Reference is now made to FIG. 3, in which the inner seal ring group 20comprises, instead of the two seal rings shown in FIG. 1 and the singlering of FIG. 2, three spaced seal assemblies 20A, 20B & 20C each havingthree seal rings in a single groove.

The outer seal ring group 19 again comprises an outermost seal 19A andtwo further seals 19B & 19C but one or more additional further sealscould be employed.

The outermost seal 19A comprises two segmented ceramic rings installedin a common groove and the inner seals each comprise a groove receivinga single ring. The ceramic rings prevent penetration of particlespresent in the working fluid into the spaces behind the ceramic rings,to provide suitable operating conditions for the inner seals 19B and19C. The ceramic seal rings allow satisfactory leakage across them oflubricant oil from the feed chamber 7.

The inner seal rings 19B and 19C have a combined scraping, oildistribution and sealing function. These rings can be of metal orsuitable polymeric material, as can the rings of the inner group 20.

The rings 19B & 19C operate in a way similar to a labyrinth seal duringthe working or discharge stroke and significantly reduce theconcentration of process fluid contamination in the lubricant oil flow.They allow the passage of sufficient lubricant for lubrication of thefront ceramic rings 19A during the discharge stroke, and during thesuction stroke, the rings provide an increased lubricant oil filmthickness to adhere to the cylinder wall surface.

The inner seal ring group 20 is designed to minimise lubricant oil flowbackwards into the feed chamber. Necessarily, process fluid that hasleaked past the seal ring group 20 will sooner or later enter the feedchamber 7 by way of the pressure intensifier outlet port 17. Anacceptable equilibrium is however reached between flow of lubricant oilwith a small degree of contamination flowing into the feed chamber 7 andthe lubricant oil flow going forward into the pressure intensifierchamber 12.

It will be evident from the description above of the overpressure orpressure intensifier mechanism operated by relative movement of thepiston head 2 and the piston rod 5, that the pressure applied to thespace between the seal ring groups 19 and 20 will vary over the cycle ofoperation of the piston pump. The pressure produced is low, or evennon-existent, at the stroke end positions, and also during the initialparts of the discharge or compression and suction strokes. As shown inFIG. 4, the overpressure mechanism can incorporate an accumulator deviceto overcome this disadvantage.

In the mechanism of FIG. 4, the fluid in the chamber 12 between thepiston head and the piston rod end portion 10 communicates with thespace between the seal ring groups 19,20, here constituted by singlerings only, by way of an axial passage 61 in the piston head from whichthe transverse passages 16 extend. The axial passage 61 includes anon-return valve 62, operating in the same sense as the non-return valve15, between the chamber 12 and the transverse passages 16. The axialpassage 61 extends beyond the transverse passages 16 into the interiorof an accumulator cylinder 64 secured on the front face of the pistonhead 2.

An accumulator piston 66 is reciprocably movable within the cylinder 64to define a variable volume accumulator chamber 67, communicating withthe passage 61. The piston 66 is biassed to restrict the volume of theaccumulator chamber 67 by a spring 69. Piston movement to enlarge thechamber is limited by a stop 70 within the cylinder. A passage 71communicates the forward end of the accumulator cylinder 64 with thefeed chamber 7 for return to the feed chamber of any lubricant oil whichleaks past the piston 66. The accumulator cylinder 64 is enclosed withinan accumulator housing 72 having a cylindrical wall closed by an endwall which functions as the working surface of the piston head 2.

The operation of the accumulator device is illustrated in FIG. 5, whichshows variations of the pressures in the accumulator chamber (P_(ACC)),the motor housing (P_(MOTOR)), and the process chamber (P_(SUCT))through an initial stroke and a subsequent normal stroke. Theaccumulator chamber 67 is charged to a certain pressure level during thedischarge stroke, the level being limited by the stop 70. When thepressure in the intensifier chamber 12 falls below that in theaccumulator chamber, the non-return valve 62 closes and the accumulatorpressure is supplied to the space between the seal rings or groups ofrings 19,20, so as to reduce substantially the variations in theoverpressure during the pumping cycle. The overpressure accumulatordevice can be employed with or without the multiple seals and barrierchamber arrangements of FIGS. 1-3.

The invention can of course be embodied in variety of ways other than asspecifically described and illustrated.

I claim:
 1. A sealing system for a piston pump comprising a piston rod (5) extending into a cylinder (1) from a motor housing (6), the motor housing containing drive means for reciprocably driving the piston rod, and the cylinder having a piston (2) reciprocable there within, the sealing system comprising a first seal means (19, 20) between the piston and the cylinder for separating a first fluid to be pumped by the piston from a second fluid around the piston rod, and second seal means (21A, 21B, 21C) between the piston rod and the cylinder separating the second fluid from a third fluid in the motor housing, wherein the second seal means comprises a first seal ring (21A), a second seal ring (21B) spaced from the first seal ring on the side thereof remote from the first seal means to define between the seal rings a first chamber (22), a third seal ring (216) spaced from second seal ring on the side thereof remote from the second seal ring to define a second chamber (27) between the second and third seal rings, means (25) for supplying a fourth fluid to the first chamber, and means (27) for maintaining a lower pressure within the second chamber than in the first.
 2. A sealing system as claimed in claim 1, wherein the first seal means comprises first and second sealing means (19A-19C & 20A-20C) and wherein means (10-12, 14-17) is provided for generating a fluid pressure in the space between the first and second sealing means, the piston and the cylinder in excess of the fluid pressures at the outer sides of the first and second sealing means.
 3. A sealing system as claimed in claim 1, wherein the means for maintaining a lower pressure in the second chamber (24) than in the first (22) comprise means (27) for evacuating or ventilating the second chamber.
 4. A sealing system as claimed in claim 1, wherein the means for maintaining a lower pressure in the second chamber (24) than in the first comprise means for supplying the fourth fluid to the second chamber at a pressure lower than that of the fourth fluid supplied to the first chamber.
 5. A sealing system as claimed in claim 4 wherein the fourth fluid is supplied to the first and second chambers (22,24) from a common source (40) through first piping (39) and is returned to the source from the second chamber through second piping (37).
 6. A sealing system as claimed in claim 5 having means for measuring the quantity of the fourth fluid entering the fluid to be pumped by measuring the quantities flowing in the first and second piping (39,37).
 7. A sealing system as claimed in claim 4, wherein the third and fourth fluids are the same and the fourth fluid is supplied to the second chamber (24) through a pressure reducing constriction (45) from the motor housing (6) or from a housing (35) for the pump.
 8. A sealing system as claimed in claim 4, wherein the second and fourth fluids are the same and the second fluid is supplied to the space around the piston rod from the first chamber through a pressure reducing restriction (14).
 9. A sealing system as claimed in claim 1 wherein the second and third fluids are the same.
 10. A sealing system for sealing between a fluid-containing cylinder (1) and a piston reciprocably moveable therein, the system comprising first and second sealing means (19, 20) operative between the piston and the cylinder at positions spaced apart along the direction of reciprocable movement, a pressure chamber bounded by the first and the second sealing means, and means (10-12, 14-17) for generating a fluid pressure in the pressure chamber (18) between the first and second sealing means, the piston and the cylinder, in excess of the fluid pressures at the outer sides of the first and second sealing means characterised in that an least one of the first and second sealing means comprises a plurality of seal rings (19A-19C; 20A-20C) spaced apart along the direction of reciprocably movement.
 11. A sealing system as claimed in claim 10, having an accumulator (64) in communication with the space (18) to regulate the pressure therein.
 12. A sealing system as claimed in claim 11 wherein the first sealing means (19) comprises a plurality of seal rings (19A-19C) of which the first (19A) on the side thereof remote from the second sealing means (20) contacts a process or working fluid being moved by the piston member (2), the first seal ring is arranged to prevent or resist passage of particles in the working or process fluid towards the or each other seal ring of the first seal means.
 13. A sealing system as claimed in claim 12 wherein the first seal ring (19A) is of ceramic.
 14. A sealing system as claimed in claim 12 wherein the first seal ring (19A) comprises plural segmented ceramic rings in a single groove in the piston member and the or each other seal ring (19B,19C) comprises a single ring of metal or polymeric material.
 15. A sealing system as claimed in claim 11, wherein the second sealing means (20) contacts lubricating oil on the side thereof remote from the first seal means (19) and comprises a plurality of seal rings (20A-20C) received in respective grooves in the piston member and spaced apart in the direction of reciprocable movement.
 16. A sealing system for sealing between a fluid-containing cylinder (1) and a piston reciprocably movable therein, the system comprising first and second sealing means (19, 20) operative between the member and the cylinder at positions spaced apart along the direction of reciprocable movement, a pressure chamber bounded by the first and the second sealing means, and means (10-12, 14-17) for generating a fluid pressure in the pressure chamber (18) between the first and second sealing means, the member and the cylinder in excess of the fluid pressures at the outer sides of the first and second sealing means, characterised by an accumulator (64) in communication with the space (18) to regulate the pressure therein.
 17. A sealing system as claimed in claim 16 wherein the accumulator (64) comprises a cylinder having a piston (66) movable therein, the piston dividing the cylinder interior into a first chamber (67) communicating with the space and a second chamber containing a spring biassing the piston.
 18. A sealing system as claimed in claim 17 wherein the accumulator (64) is located at an end face of the piston and communicates with the space (18) by axially and radially extending passages.
 19. A sealing system as claimed in claim 18 having a passage (71) communicating the second chamber through the end face of the piston remote from the accumulator.
 20. A sealing system as claimed in claim 16 wherein the piston comprises relatively reciprocable first and second parts (2,10) having between them a variable valance cavity (12) communicating with the space, the excess pressure being generated by relative movement of the first and second parts, and wherein the accumulator (64) communicates with the cavity through a check valve (62) permitting fluid flow into the accumulator.
 21. A sealing system as claimed in claim 20 wherein the first and second seal means are both carried by one (2) of the two relatively reciprocable members.
 22. A sealing system as claimed in claim 20 wherein the first part (2) contacts a process or working fluid to be moved by the piston member and is driven by the second member (10).
 23. A sealing system as claimed in claim 22 wherein the accumulator (64) is carried by the first part (2) at the side thereof contacting the process or working fluid. 